1. Field of the Invention
The present invention relates to a noise filter for cancelling noises of an ac power supply, more particularly, to a noise filter in which a housing for an additional earth inductor is defined within a filter body, thereby enabling the earth inductor to be established in the filter body and ensuring easy connection of the earth inductor to associated lead wires.
2. Description of the Prior Art
As is well-known, a typical ac line filter is used for cancelling noise produced in the ac power supply. FIG. 4 shows such an ac line filter of which windings or inductor coils are provided on a closed circular magnetic path.
In FIG. 4, and FIG. 5 depicting the equivalent circuit of FIG. 4, the ac line filter has a pair of windings 4,4' which are connected to terminals 1,1' and 2,2'. A core 3 forming the circular closed magnetic path has effective permeability higher than a certain value over a wide frequency range, from a low frequency region to a desired high freqeuncy region. The windings 4,4' are wound in a direction opposite to each other at opposing locations on the closed magnetic path so that a common mode current of a high frequency which flows into the terminals 1,1' or 2,2' may cause a magnetic flux in the same direction.
Capacitors 5,5' bypass a normal mode current of a high frequency in opposite phase which flows into the filter through the terminals 1,1' or 2,2'. Capacitors Cg bypass the high frequency common mode current flowing into the terminals 1,1' or 2,2' to the ground through a ground terminal 7. An electrically conductive member 6 is connected to, for example, a metal chassis or other associated metal components mounted close to the line filter.
Conventionally, such a noise filter is assembled in a metallic filter housing having a plug assembly on which terminals, for example, such as terminals 1,1' or 2,2' are securely mounted. In facing with a rear surface of the plug assembly of the filter housing, a filter body is defined to receive filter elements, for example, the windings(i.e., inductor coils) 4,4 wound on the core 3, the capacitors 5,5 serving as X-type capacitors and the capacitors Cg serving as Y-type Capacitors. A metallic bottom plate is joined to the bottom surface of the filter body.
An inner cover is disposed on the upper Slide of the filter body and the metallic fillter housing having an opened bottom surface covers the entire plug assembly and the inner cover.
Also, an additional earth inductor is connected to the periphery of the bottom plate of the noise filter.
With such a noise filter housing constructed as mentioned above, however, it is difficult to provide a space for mounting the additional earth inductor within the filter body. For this reason, the additional earth inductor must be established on the periphery of the bottom plate. Also, when the additional earth inductor is connected to associated lead wires, it generally needs a complicated connection structure between the additional earth inductor and the associated lead wires. Moreover, since the additional earth inductor extruded extends from the bottom plate, not only it is difficult to position the additional earth inductor on the bottom plate, but also the aesthetic sense of the noise filter is injured undesirably.